In clinical practice, because of increasing demand of environmental degradation of plastics recently Biodegradable TPEs have been paid increasing attention. These polymers have broad application in bio-medical fields, such as surgical sutures, matrices for drug delivery and scaffold in tissue engineering.

To gain the elasticity possessed by many tissues and organs these polymers are being recently developed by many authors. In this study an approach for making such type of biodegradable thermoplastic elastomer (BTPE), by blending PLCA with PLGA is discussed.

Introduction :

Biodegradable materials such as polyactic acid (PLA), poly (glycolic acid) (PGA), poly (Î-caprolactone) (PCL) and poly (1,4 – dioxan 2 – one) (PPDO) are used in temporary therapeutic applications such as sustained drug delivery, surgery and tissue engineering. But these materials available today as plastics with a high young’s modulus and the elongation at break – values. As a consequence of this, they are inappropriate for clinical uses that require strong, flexible, bio-degradable polymeric materials such as ureters artificial skin and veins.

PLA is undoubtedly one of the most important biodegradable materials. But due to certain shortcomings including a low degradation rate, rigorous processing conditions and hydrophobicity its applications are restricted greatly. Similarly PCL is another important biomaterial with outstanding permeability to drugs and flexibility; however its high degree of crystallinity at room temperature and its low melting point limit its use.In order to overcome the shortcomings of each of these materials a copolymer PLCA (Poly (DL-lactide-co-Î-caprolactone) is made from PLA and PLC. PLCA combines the biodegradability of PLA with the flexibility of PCL. More notable, in contrast with most biodegradable plastics, PLCA is a type of elastomer, which can be used to manufacture artificial skin, degradable ureters and veins. Although PLCA clearly broadens the potential applications of biodegradable materials, studies show that it exhibits excellent flexibility but relatively poor tensile strength.

Many technologies such as co-polymerization, reinforcement with nano fillers and fibres and cross-linkages have been developed to improve the mechanical developed to improve the mechanical properties of PLCA. But it has been recently found that when PLCA is blended with PLGA, another imported copolymer of PLA and PGA desired improvement in mechanical properties of PLCA could be possible. PLGA is also a widely used biomaterial with excellent mechanical properties.

Stress-Strain behaviour of blends (BPTE)

When stress-strain behaviour of PLCA/PLGA blends are evaluated using a electro-mechanical universal testing machine with a tensile rate of 10 mm/min., the effect of  mass fraction of PLGA on the mechanical properties of blends can be observed in accordance with the pattern as given in Fig. 1.

Fig 1.The effect of mass fraction of PLGA on the mechanical properties of blends

It is found that PLCA shows a low tensile strength of 0.92 MPa. Whereas PLGA shows a much higher tensile strength of 46.1 MPa. With an increase in PLGA content, tensile strength of the blends increases gradually. For instance, the blend with 40% PLGA exhibits a high tensile strength of 6.6 MPa which is more than 7 times as much as that of virgin PLCA. Fig. 1 also shows the elongation at break of the blends with various PLCA/PLGA ratios. PLCA shows a large elongation at break of more than 650%.

It is also observed that the elongation at break of blends decreases dramatically with an increasing amount of PLGA. Comparison of Young’s modulus among PLCA, PLGA and all blends is also shown in Fig. 1. It shows that PLCA exhibits a low Young’s modulus of less than 1MPa at the test temperature of 23OC and PLGA shows a much higher Young’s modulus of 670 MPa which is approximately 700 times as much as that of PLCA. With an increased PLGA content in blends, the Young’s modulus
arugments markedly from 0.9 MPa to 670MPa.

Standard Composition for BTPE

The results from the stress-strain test reveals that the addition of PLGA in blends leads to remarkable change in the chemical properties. But from this result Thermoplastic elastomer behaviour of blends cannot be confirmed. This improvement only indicates that with an increase in PLGA content, tensile strength and Young’s modulus increases markedly while elongation at break decreases. This suggests that there is sufficient stress transfer across the PLCA and PLGA interface during test. It is also notable that too much PLGA in blends can cause materials to transform a elastomer to a tough plastic. Therefore, the mass fraction of PLGA should be controlled in such a way that TPE behaviour of blends can observed.

Ascertaining TPE behaviour

In order to qualify as a thermoplastic elastomer, a suitable blends of PLCA/PLGA must have the ability to be stretched to moderate elongation and upon the removal of stress, return to something close to its original shape. This is called tensile recovery property of the material.

This is done through cyclic test. The measured degree of recovery of a typical TPE could be more or less in the manner as demonstrated in Fig. 2.

Fig. 2 Cyclic test of a typical TPE at room temperature

Which indicates that the recovery is 82.1% when a typical TPE is stretched upto 100%. The recovery is 70% when it is stretched upto 200%. Sometimes, when time is  given, the recovery could be more even after higher percentage of elongation or stretching. When the typical TPE stretched upto 300% the final measured recovery after  10 minutes goes upto 89.7%. Therefore, biodegradable blend at a particular composition if show properties similar to that of vulcanized rubber at service temperature, then that should be termed as biodegradable TPE (BPTE).

Conclusion :

From the above studies it is found that there could be a possibility of getting suitable blend composition of PLCA/PLGA with tensile strength as high as 40MPa and elongation at break of 600% which could be very soft, yet have good strength. This may find applications in biomedical fields such as soft tissue engineering and artificial skin.

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